A system has been developed which determines the consciousness-degraded state of the driver of the vehicle due to, for example, fatigue, drowsiness, and inattentive driving and outputs a warning to the driver. As a method for determining the consciousness-degraded state, for example, there is a method which determines the drift of the vehicle in the lane as a unique steering pattern which appears in the consciousness-degraded state. Patent Literature 1 discloses a device which includes image recognition and detection means for recognizing a lane on the basis of the image of the front side of the vehicle which is captured by an in-vehicle camera and detecting the position of the vehicle relative to the lane and steering angle detection means for detecting the steering angle of a steering wheel and determines drift using a combination of the two detection means.